The Nuclear and Industrial Safety Agency in Japan has doubled its estimate of the radiation released by the Fukushima nuclear plant in the aftermath of the massive earthquake and tsunami.
It has also admitted the meltdown in two of the reactors was much faster than originally thought.
NISA says it believes the Fukushima plant emitted almost 800,000 terabecquerels of radioactive material into the air in the days after it was hit by a 9.0-magnitude earthquake and massive tsunami on 11 March.
The ABC reports that is more than double the original estimate. It is based on new information suggesting the No 1 and No 2 reactors suffered meltdowns much earlier than thought.
The revision reveals the failure to contain the disaster resulted in much more radioactive contamination of the soil, sea and air than the plant's operators had acknowledged.
The disaster is rated a maximum seven on the international nuclear accident scale - the same level as the Chernobyl meltdown 25 years ago.
In late May, it was feared two workers at the crippled plant in north-east Japan were exposed to radiation above the level allowed in emergencies.
Japanese media have reported that Tokyo Electric Power Company workers were exposed to at least 250 millisieverts of radiation.
The ABC reports the maximum dose used to be 100 millisieverts, but it was raised to 250 after the nuclear crisis began.
It is believed the men were exposed while working in the plant's central control room and outside the reactor buildings.
Internal exposure occurs when people take radioactive substances into their bodies through tainted air or food and drink.
The workers were reportedly found with levels of radioactive iodine in their thyroid glands 10 times higher than that of their fellow workers.
TEPCO says it has been measuring the internal exposure to radiation of all employees involved in emergency work at the Fukushima plant.